1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of reducing feedback load and feedback overhead in a multi-cell cooperative network and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy requirement of IMT-Advanced on capacity and cell-edge throughput. The LTE system includes anew radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved NodeBs (eNBs) which is a transmission point for communicating with multiple UEs, and communicates with a core network including a mobility management entity (MME), a serving gateway, etc., for Non Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multi-point transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must be compatible with the standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
When a UE and multiple transmission points (e.g. an eNB, a relay node or a remote antenna of a transmission point) are configured in the CoMP scheme, the UE may communicate with the transmission points simultaneously, i.e., access a service via all or part of the transmission points. These transmission points form a CoMP cooperating set. More specifically, an eNB is one of transmission points, and it may manage multiple transmission points (e.g. via remote radio head (RRH)). That is, Cell IDs of different transmission points may be different (e.g. when being managed by different eNBs), or may be the same (e.g. when being managed by the same eNB). Thus, signals transmitted between the UE and the transmission points can be easily recovered due to better quality of the received signals. In detail, when the transmission points are involved in the CoMP scheme, one of the transmission points is a serving point (e.g. serving cell). In general, link quality between the serving point and the UE is better than those between other transmission points and the UE. Control information required for the CoMP scheme is usually transmitted by the UE to the serving point first. Then, the serving point exchanges the control information with other transmission points such that the CoMP scheme can operate regularly. Further, the CoMP scheme can be classified into two main categories: Joint Processing (JP) and Coordinated Scheduling/Beamforming (CS/CB). When the JP is configured (i.e. enabled), data for the UE is available at and transmitted from part or all of the transmission points. When the CS/CB is configured, the data for the UE is only available at and transmitted from one of the transmission points of the CoMP cooperating set. The JP can be further divided into two categories: joint transmission and dynamic point selection. When the joint transmission is configured, the data for the UE can be transmitted from multiple transmission points (e.g. coherently or noncoherently) to the UE to improve received signal-to-interference-plus-noise ratio (SINR). When the dynamic point selection is configured, the data for the UE is transmitted from only one of the transmission points (e.g. according to the decisions made in coordination among the transmission points) to the UE to improve signal quality and/or reduce the interferences. On other hand, when the CS/CB is configured, the data for the UE is only transmitted from one of transmission points of the CoMP cooperating set to the UE, while other transmission points may stop transmissions or adjust beamforming to mitigate the interferences.
However, channel information between the transmission points and the UE should be available at the transmission points, before the CoMP scheme can be operated efficiently and properly. Accordingly, the UE should feed back the channel information to the transmission points periodically or aperiodically. Preferably, the channel information is related to channel condition between the UE and the transmission points. However, an amount of the channel information increases with a size of the CoMP cooperating set (i.e., an amount of the transmission points). That is, when the size of the CoMP cooperating set increases, an amount of radio resources required by the UE to feed back the channel information also increases. Not only feedback load of the mobile device is increased, but a large amount of feedback overhead is caused on an uplink between the mobile device and the transmission points and in backhauls between the transmission points. Thus, the CoMP scheme cannot be operated regularly when uplink feedback resource is limited. Therefore, how to solve the problem of reduction of the feedback load and the feedback overhead is a topic to be discussed and addressed.